This invention relates to precision measurement and more particularly to precision measurements of internal dimensions over a broader range than the standard screw thread micrometer head is capable of handling.
The prior art includes numerous devices for making precision measurements by preseting the screw-thread micrometer head to measure the item of the size at hand. One device for making external measurements only, as shown in U.S. Pat. No. 515,235, requires the use of rods of a calibrated length. In this device micrometer head J is set to zero, calibrated rod H is held by hand between anvils C and K while moveable arm D is slid along rail A to close anvils C and K on rod H and then set-screw a' is tightened to hold moveable arm D in place on rail A. This device, because of the hand setting method and the wear on key K and rail A does not provide for repeatable measurements of equivalent accuracy each time.
Another approach in the prior art has been to accurately position a moveable jaw with respect to a fixed jaw by means of pre-positioned posts (U.S. Pat. No. 3,807,049) for external measurements only or holes (U.S. Pat. No. 2,469,926). These also present a problem. Namely, the accurate placement of the pre-positioned posts or holes. Additionally, these present serious problems as a result of normal wear which will result in greatly reduced accuracy of these devices.
A third approach utilizes a micrometer head mounted on a hand positional carriage (U.S. Pat. No. 1,425,027). This lacks precision and the ability to make repeatable measurements since carriage 3 is pre-positioned by aligning the selected index mark 17 to index mark 16 merely by eye. Once carriage 3 is fastened in position by set-screw 15. This, as was the device specified in the first above-referenced patent, is also subject to loss of accuracy due to wear.
What is needed is a precision device which is easy and fast to use without suffering from the problems of wear, inaccurate placement of reference posts or holes, or simple alignment by eye. It is believe that the present invention is such a device.